token_metadata: Do nothing if tokens is empty.
token_metadata: Do nothing if tokens is empty.

When replacing a node, we might ignore the tokens so that the tokens is
empty. In this case, we will have

   std::unordered_map<inet_address, std::unordered_set<token>> = {ip, {}}

passed to token_metadata::update_normal_tokens(std::unordered_map<inet_address,
std::unordered_set<token>>& endpoint_tokens)

and hit the assert

   assert(!tokens.empty());

avikivity/scylla,raphaelsc/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,kjniemi/scylla,duarten/scylla,kjniemi/scylla